1. Field of the Invention
The present invention relates to a method for producing glycine. More particularly, the present invention is concerned with a method for producing glycine, which comprises subjecting an aqueous solution of glycinonitrile to a hydrolysis reaction in a hydrolysis reaction system under the action of a microbial enzyme, thereby converting the glycinonitrile to glycine while by-producing ammonia, wherein the hydrolysis reaction system contains at least one organic impurity compound inhibiting the microbial enzyme, wherein the organic impurity compound has a molecular weight of 95 or more and contains a specific structure, and wherein the hydrolysis reaction is performed under conditions wherein, during the hydrolysis reaction, the content of the organic impurity compound inhibiting the microbial enzyme in the hydrolysis reaction system is maintained at a level of 10% by weight or less, based on the weight of the hydrolysis reaction system. By the use of the method of the present invention, a high purity glycine which is useful as a food additive and as a raw material for synthesizing pharmaceuticals, agricultural chemicals and detergents can be produced easily and efficiently on a commercial scale without causing a heavy burden on the environment.
2. Prior Art
Conventionally, glycine has been produced by a process which comprises: synthesizing glycinonitrile from formaldehyde, hydrogen cyanide and ammonia by the Strecker method; converting the synthesized glycinonitrile into glycine soda and ammonia by hydrolysis using an alkali (such as caustic soda); neutralizing the glycine soda with an acid (such as sulfuric acid) to obtain glycine; and recovering the glycine by crystal-deposition (see Unexamined Japanese Patent Application Laid-Open Specification Nos. Sho 43-29929, Sho 51-19719, Sho 49-14420 and Sho 49-35329). As apparent from the above, such a conventional hydrolysis method employing a base uses an alkali and an acid in amounts each equivalent to the amount of glycine produced. Therefore, such method has a problem in that large amounts of salts are by-produced and the disposal of the by-products causes a heavy burden on the environment. Further, since the solubilities of the by-produced salts are similar to that of glycine, the recovery of glycine cannot be achieved by a single-step crystal-deposition and, therefore, a cumbersome operation, such as repetition of a cycle comprising crystal-deposition and circulation of a mother liquor, is necessary for recovering glycine (see Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-34113 (corresponding to DE 2541677-B and NL 7511023-B)). In addition, an aqueous glycinonitrile solution as an intermediate is unstable at a pH of 2.5 or more, and it is known that the higher the temperature, the greater the likelihood that glycinonitrile suffers denaturation, such as decomposition and discoloration (see Unexamined Japanese Patent Application Laid-Open Specification Nos. Sho 49-14420, Sho 54-46720 and Sho 54-46721). In “Kogyo Kagaku Zasshi (Journal of Industrial Chemistry)”, Volume 70, page 54 (1967) published by Japanese Chemical Society, Japan, it is stated that hydrogen cyanide is likely to be denatured by polymerization under alkaline conditions, and black solids are generated as the polymerization proceeds. Further, “Jikken Kagaku Kouza (Lectures on Experimental Chemistry)”, 1st ed., page 347, published by Japanese Chemical Society, Japan, describes that a cyanomethyl group of glycinonitrile and the like is likely to be denatured by addition polymerization under alkaline conditions, resulting in the generation of pyridine compounds and pyrimidine compounds. Unexamined Japanese Patent Application Laid-Open Specification No. Sho 62-212357 (corresponding to U.S. Pat. No. 4,661,614) discloses that an imine compound, such as iminodiacetonitrile, can be synthesized from formaldehyde, hydrogen cyanide and ammonia. Examined Japanese Patent Application Publication Specification No. Sho 51-244815 discloses that when glycinonitrile is heated, glycinonitrile generates ammonia and imine compounds (such as iminodiacetonitrile), and further heating causes the imine compounds to denature, resulting in the generation of black compounds. Therefore, in the conventional methods, a lowering of the yield of glycine due to the above-mentioned decomposition and denaturation is unavoidable. Further, the conventional methods have a defect in that a cumbersome treatment employing an activated carbon or a special ion exchange resin is necessary for removing a discolored matter (see Unexamined Japanese Patent Application Laid-Open Specification No. Hei 3-190851 and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-226949 (corresponding to EP 459803-B)).
As a method for hydrolyzing glycinonitrile under moderate conditions without using large amounts of an alkali and an acid, there is known a method in which glycinonitrile is simply hydrolyzed using a microorganism having the activity to hydrolyze a nitrile group, thereby obtaining glycine and amxnonium. Examined Japanese Patent Application Publication Specification No. Sho 58-15120 (corresponding to French Patent No. 225585) discloses a method in which a hydrolysis reaction is conducted using Brevibacterium R312 suspended in a reaction medium liquid which has been adjusted to have a pH value of 8 with caustic potash or the like. Examined Japanese Patent Application No. Hei 3-62391 (corresponding to EP 187680-B) discloses a method in which a hydrolysis reaction is conducted using Corynebacterium N-774 suspended in a reaction medium liquid which is a phosphate buffer having a pH value of 7.7. Unexamined Japanese Patent Application Laid-Open Specification No. Hei 3-280889 (corresponding to EP 450885-B) discloses a method in which glycine is produced from glycinonitrile by using a microorganism belonging to the genus Rhodococcus, Arthrobacter, Caseobacter, Pseudomonas, Enterobacter, Acinetobacter, Alkaligenes, Corynebacterium or Streptomyces which is capable of hydrolyzing a nitrile group, wherein the microorganism is suspended in a reaction medium liquid which is a phosphate buffer having a pH value of 7.7. However, as shown in the working examples of these patent documents, these methods require that a lyophilized microorganism be used in an amount which is equal to or greater than the amount of glycine, or alternatively, the reaction be performed for 30 hours using a large amount of a lyophilized microorganism, namely 5% by weight or more, based on the weight of glycine. Further, as also shown in the working examples of these patent documents, these methods require that a neutralizing agent be successively added to the reaction system to maintain the pH of the reaction system within the neutral range so as to maintain the activity of the microorganism. In general, for neutralizing an ammonium salt of glycine, sulfuric acid or phosphoric acid is added to the reaction system, and hence a large amount of ammonium sulfate or ammonium phosphate is likely to remain in the reaction system. Therefore, the above-mentioned methods for producing glycine by using a microorganism are disadvantageous in that a large amount of an acid must be used and a large amount of waste must be discarded. Further, the abovementioned methods using a microorganism have the following problems. In the above-mentioned methods using a microorganism, the operation for recovering glycine needs a step of adding methanol or the like in addition to the concentration step, so that the operation for recovering glycine becomes complicated, as compared to the case of the alkaline hydrolysis method mentioned above (see Examined Japanese Patent Application Publication Specification No. Sho 58-15120 (corresponding to French Patent No. 225565)) . Further, the above-mentioned methods use a large amount of a microorganism, and this results in a further increase in the amount of waste. On the other hand, a method is known which employs a microorganism and electric dialysis and in which glycine and ammonia are separately recovered while recycling an alkali (see Unexamined Japanese Patent Application Laid-Open Specification No. Sho 10-179183 (corresponding to U.S. Pat. No. 5,932,454 and EP 852261-A)). This method is a method for producing glycine, comprising the following steps: a step of producing amrnonium salts of organic acids (including glytine) by using a microorganism; a step of converting the amrnonium salts into alkali salts, thereby eliminating ammonia; a step of recovering the eliminated ammonia; a step of separating the alkali and the organic acids from each other by electric dialysis; a step of extracting the organic acids with an organic solvent; and a step of separating the organic acids from the organic solvent. In the working examples of the above-mentioned patent document, only 0.3 mole of an organic acid is obtained from the microorganism cultured in 20 g of glycerin medium, showing that the activity of the microorganism is very low. Thus, this method has disadvantages in that multiple steps and cumbersome operations are necessary, a large amount of electricity is consumed, and a large amount of a microorganism is used and discarded.
As apparent from the above, the conventional methods for producing glycine from glycinonitrile by the use of a microorganism have disadvantages in that both the activity per unit amount of a lyophilized microorganism and the activity per unit time are low, and large amounts of culture medium and microorganism must be discarded. In addition, in the case of a method in which the electric dialysis is not employed, there are problems in that the recovery of ammonia is difficult due to the use of a neutralizing agent for adjusting the pH of the reaction system or for recovering glycine, and that a step of removing the neutralizing agent is necessary. Even in the case of a method in which the electric dialysis is employed, the recovery of ammonia requires not only electricity but also a cumbersome, multiple-step operation. Therefore, the commercial practice of this method cannot be satisfactorily performed.